1. Field of the Invention
The invention relates generally to the field of communications. More particularly, the invention relates to spread-spectrum communications.
2. Discussion of the Related Art
In standard direct-sequence spread-spectrum (DSSS) methods, each doubling of the number of users in a given frequency channel decreases the reliability by 3 dB due to code interference. In the typical DSSS systems, carrier separation is based on the chip rate as discussed in the following patent summaries.
A technique for utilizing orthogonal frequency spacing to enhance communication efficiency is disclosed in Natali, U.S. Pat. No. 5,623,487, “Doubly Orthogonal Code and Frequency Multiple Access Communication System” the entire contents of which are hereby expressly incorporated by reference herein for all purposes. Natali discusses orthogonal code division multiple access (OCDMA) radio communication systems having at least one base station and a plurality of remote subscriber terminals, the a method of reducing the sensitivity of OCDMA to access noise due to time base error and multi-path delay spread comprising: (1) reducing the size of the orthogonal signal set on a single carrier, and (2) providing additional carriers with orthogonal frequency spacing for additional subscriber capacity. The orthogonal frequency spacing of Natali is based on the spread-spectrum chip rate as discussed in the following excerpt. “The modulated carrier frequency is selected from one of N frequencies which are orthogonal over a RW (Radamacher-Walsh) chip interval, i.e. the carrier frequencies are spaced by the RW chipping rate. The composite signal is up-converted to the appropriate frequency band for transmission.”
A similar technique is disclosed in Kondo, et al., U.S. Pat. No. 5,521,937, “Multicarrier Direct Sequence Spread System and Method,” the entire contents of which are hereby expressly incorporated by reference herein for all purposes. Kondo discloses an orthogonal frequency spacing based on the chip rate as discussed in the following excerpt: “The M carriers are designed to be orthogonal to each other. That is,∫OT cos(ωit+φi)cos(ωjt+φj)dt=0 for i≠j  , (Eq. 1)where T is the bit duration, and ωi and ωj are different carrier frequencies for i≠j. Orthogonality is achieved by choosing
                                          ω            i                    =                                                    m                ⁢                                  π                  T                                            +                                                (                                      i                    -                    1                                    )                                ⁢                n                ⁢                                                      4                    ⁢                                                                                  ⁢                    π                                    T                                                      =                                          m                ⁢                                  π                  T                                            +                                                (                                      i                    -                    1                                    )                                ⁢                                                      4                    ⁢                                                                                  ⁢                    π                                                        T                    c                                                                                      ,                            (                  Eq          .                                          ⁢          2                )            where m is an integer, n is the number of chips per bit [that is, the rate at which the bits of the data signal d(t) are chopped into chips], and Tc is the chip duration.”
Another technique that uses chip-rate frequency spacing to achieve orthogonality is disclosed in Schilling, U.S. Pat. No. 5,274,665, “Polyopoly Overlapping Spread Spectrum Communication System and Method,” the entire contents of which are hereby expressly incorporated by reference herein for all purposes. Schilling discloses a system such as discussed in the following excerpt: “The carried signal of the second transmitter station is shifted in frequency from the carrier signal of the first transmitter station by the chip rate of the message-chip-code signals. The carrier signal of the third transmitter station is shifted in frequency from the carrier signal of the first transmitter station by twice the chip rate of the message-chip-code signals, etc.”
A problem with these technologies is user interference due to channel overcrowding. As the number of users increases, this problem is exacerbated.
Modern spread-spectrum communication systems, both CDMA and FDMA, are reaching the limit of user saturation in populated urban areas. The usual solutions of allocating more bandwidth with a multiplicity of standards and frequency channels are unattractive due to the hardware cost to the vendor and the proliferation of specialized consumer units required. To take a particular example, consider the Industrial, Scientific, and Medical (ISM) band of 902 to 928 MHz that is commonly used for unlicensed communications, the standard U.S. IS-95 cellular telephone bands of 824–849 MHz (transmit) and 869–894 MHz (receive), or the PCS bands from 1850–1910 MHz (transmit) and 1930–1990 MHz (receive). A typical ISM vendor will choose a 915-MHz carrier for DSSS or frequency-hopping spread-spectrum (FHSS) CDMA (code-division multiple-access) channels. Each user is assigned a pseudo-random code such that (few) users active on the same carrier frequency have little probability of interfering with one another. The code itself is unambiguous to decode, allowing effective and noise-free transmission of digital information (usually) at baud rates consistent with high-quality voice communication.
Since these codes appear to be random, one interferes with another by appearing as random noise. If two users are transmitting at the same power level (as seen by a receiver) using one of the same signals or codes, the ability to decode the desired signal in an error-free manner will degrade by 3 dB. If the number of interferers is doubled to 2 (three transmissions on the same channel), the effective signal-to-noise degradation will be by 6 dB. In a similar manner, for each doubling of users that generate noise-like random signals as far as decoding a particular signal is concerned, the ability to detect and use the desired signal will degrade by an additional 3 dB.
A simplistic solution to this problem is to allocate a new frequency channel once the previous one has reached capacity as defined by a certain level of inter-user interference. The drawbacks to this simplistic approach are twofold: (1) such allocations must be coordinated with the services presently extant and (2) the particular band is soon full; e.g., the 915-MHz ISM band can only support 14 such channels if the chipping rate is 1 MHz.
Other solutions to relieve this overcrowding have been known for many years and are discussed in such texts as the one by Dixon(1) and Peterson(2) under the rubric “hybrid methods.” The central idea of these solutions is to frequency hop between DSSS channels. The standard discussion of hybrid FDMA/CDMA systems is from the processing-gain standpoint. Various authors have correctly and consistently pointed out that the increase in processing gain to be expected from any such hybrid system is approximately 3 dB in a typical case.
What is missing in these prior art discussions that start with a predetermined number of users and frequency-hop channels is a method that accommodates a probabilistic or deterministic increase in the number of users normalized to a standard bandwidth (bits/sec/Hz, for example). What is needed, therefore, is an approach that accommodates an increase in the number of normalized users.
What is also missing in these prior-art discussions are methods for optimizing the loading or equalization of the users across multiple, closely spaced frequency channels within the allocated band. What is also needed, therefore, is an approach to optimizing the loading or equalization of the users across multiple, closely spaced frequency channels within the allocated band.
Heretofore, the requirements of accommodating an increase in the number of normalized users and optimizing the loading of users across multiple, closely spaced frequency channels have not been fully met. What is needed is a solution that addresses both of these requirements.